The present invention relates to a toner for developing an electrostatic latent image, a process for producing the same, a developer for developing an electrostatic latent image, and a process for forming an image in an electrophotographic process and an electrostatic recording method.
In the electrophotographic process, an electrostatic latent image formed on a latent image holding member (photoreceptor) is developed with a toner containing a coloring agent, and a resulting toner image is transferred to a transferring material and then fixed with a heat roll, so as to obtain an image. The latent image holding member is separately subjected to cleaning for forming another electrostatic image.
A dry developer used in the electrophotographic process is roughly classified to a one-component developer solely using a toner containing a binder resin and a coloring agent and a two-component developer containing the toner mixed with a carrier. The one-component toner can be further classified to a magnetic one-component type, in which magnetic powder is used and the developer is transported by a developing roll with a magnetic force, and a non-magnetic one-component type, in which magnetic powder is not used and the developer is transported by a developing roll with application of charge by a charging roll.
Since a second half of the 1980s, an apparatus of a compact size and sophisticated performance is demanded in the market of electrophotography based on a trend of digitization, and particularly with respect to quality of a full color image, high class printing and high image quality equivalent to a silver halide photography. A digitized process is indispensable as means for attaining high image quality, and an effect of the digitization in image quality includes the complicated image processing that can be conducted at a high speed. By employing such a digitized process, text information and photographic image information can be controlled separately, and thus the reproducibility of the quality of both of them is greatly improved in comparison to the analog technology. Particularly, with respect to a photographic image, it is notable that gray level correction and color correction can be conducted, and the digitized process is advantageous over the analog technology in gray level characteristics, fineness, sharpness, color reproducibility and graininess. However, a latent image formed by an optical system must be faithfully reproduced as an image output, and therefore an attempt of realizing faithful reproduction is increasingly conducted with the decrease in particle diameter of a toner. However, it is difficult to stably obtain high image quality only by decreasing the particle diameter of the toner, and there is increasing importance in improvement of basic characteristics of development, transferring and fixing characteristics.
In particular, a color image is formed by superimposing color toners of three colors or four colors. Therefore, when at least one of the toners exhibits different performance from the initial stage or different performance from the toners of the other colors from the standpoint of development, transferring or fixing, deterioration in image quality occurs, such as deteriorated color reproducibility, low graininess and color unevenness. It is an important demand to maintain an image having stable high image quality equivalent to the initial stage even after the lapse of time that the characteristics of the toners are stably controlled. It has been reported that a toner is agitated in a developing device, and the fine structure on the surface of the toner is easily changed, to cause great change in transferring property (JP-A-10-312089).
In recent years, a cleaning system without cleaner has been proposed from the standpoints of miniaturization of an apparatus for space saving, decrease of the waste toner for environmental protection, and prolongation of the service life of the latent image holding member. In the cleaning system without cleaner, without using a cleaning system, the toner remaining on a photoreceptor drum after transferring is dispersed by a brush in contact with the photoreceptor drum, and the dispersed toner is recovered by the developing device simultaneously with development (JP-A-5-94113). In general, when the remaining toner is recovered simultaneously with development, because the recovered toner has different charging characteristics from the other toners to cause problems in that the recovered toner is not developed but is accumulated in the developing device, it is necessary that the transferring efficiency is further improved to control the amount of the recovered toner to the minimum value.
It is proposed to make the shape of the toner approaching a sphere shape to improve the flowability, the charging property and the transferring property (JP-A-62-184469). However, the following problems occur when the toner has a sphere shape. A developing device is equipped with a transporting amount controlling plate for controlling the transporting amount of the developer constant, and it can be controlled by changing the distance between a magnet roll and the transporting amount controlling plate. However, when a toner having a sphere shape is used, the flowability of the developer is increased, and at the same time, the tapped bulk density thereof is increased. As a result, a phenomenon occurs in that the developer piles up at a part where the transportation thereof is controlled, and the transporting amount becomes unstable. While the transporting amount can be somewhat improved by controlling the surface roughness of the magnet roll and making the distance between the controlling plate and the magnet roll small, the packing phenomenon due to piling up of the developer is becomes remarkable to increase the stress applied to the toner. A problem has been confirmed in that, owing to the phenomenon, the change of the micro structure of the surface of the toner, particularly burying and peeling of an external additive, readily occurs, and thus the developing property and the transferring property are greatly changed from those in the initial stage.
In order to solve the problems, it has been reported that the packing phenomenon is suppressed by using a spherical toner and a non-spherical toner in combination to attain high image quality (JP-A-6-308759). However, although the packing phenomenon is effectively suppressed, the non-spherical toner is liable to remain as a transferring residue, and a high transferring efficiency cannot be attained. Furthermore, in the case where the simultaneous recovering of the developer is conducted, there is a problem in that the non-spherical toner as the transferring residue is recovered to increase the proportion of the non-spherical toner, and the transferring efficiency is further decreased.
There has been disclosed that in order to improve the developing property, the transferring property and cleaning property of a spherical toner, two kinds of inorganic fine particles, one of which has an average particle diameter of 5 mxcexc or more and less than 20 mxcexc, and the other of which has an average particle diameter of from 20 to 40 mxcexc, are used in combination, which are added in specific amounts (JP-A-3-100661). While this method provides excellent developing property, transferring property and cleaning property in the initial stage, because the stress applied to the toner cannot be reduced after the lapse of time, burying and peeling of an external additive readily occurs to greatly change the developing property and the transferring property from those in the initial stage.
It has been disclosed that the use of inorganic fine particles is effective to suppress the burying of the external additive on the toner (colored particles) due to the stress (JP-A-7-28276, JP-A-9-319134 and JP-A-10-312089). However, since the true specific gravity of the inorganic particles is large, the peeling of the external additive becomes unavoidable due to the stress of agitation in the developing device when the external additive particles become large. Furthermore, because the inorganic particles do not have a complete spherical shape, it is difficult to control the standing of the external additives to a constant extent when it is adhered on the surface of the toner (colored particles) . Accordingly, unevenness occurs in the microscopic shape of surface unevenness functioning as a spacer, and the stress is selectively applied at the protruded parts, whereby the burying and peeling of the external additive is further accelerated.
There has been disclosed that organic fine particles of from 50 to 200 nm are added to the toner (colored particles) to effectively manifest the function of spacer (JP-A-6-266152). By using spherical organic fine particles, the function of spacer can be effectively manifested in the initial stage. However, although the organic fine particles exhibit less burying and peeling on application of stress of lapse of time, the organic fine particles themselves are deformed, and thus the high function of spacer cannot be stably manifested. Furthermore, it can be considered that a large amount of the organic fine particles are adhered on the surface of the toner (colored particles), or in alternative, organic fine particles having a large particle diameter are used, but in such cases, the characteristics of the organic fine particles are largely reflected. That is, adverse affects on charging and development occurs in that the powder characteristics of the toner added with inorganic particles are adversely affected, i.e., the flowability and the thermal cohesiveness are deteriorated, and the freedom of controlling the charging property is lowered because the organic fine particles themselves have a charge application function.
In recent years, there are great requests on color printing, particularly on-demand printing, and a method has been reported in that a multi-color image is formed on a transferring belt for high-speed duplication, and the multi-color image is transferred to a image fixing material at a time, followed by fixing (JP-A-8-115007). In this method, transferring is repeated twice, i.e., the primary transferring from a photoreceptor to the transferring belt and the secondary transferring from the transferring belt to a transferring material, and as a result, the importance of the technique for improving the transferring efficiency is increased. Particularly in the secondary transferring, the multi-color image is transferred at a time, and the conditions of the transferring material (such as the thickness and the surface property of paper) variously changed, the charging property. Thus, in order to suppress the influence thereof, the developing property and the transferring property are necessarily controlled in a precise manner.
It has also been disclosed a technique in that respective color images are transferred to an intermediate transferring material and then subjected to simultaneous transferring and fixing on a transferring material for saving the consuming electric power and the space and for obtaining an image having high image quality. (JP-A-10-213977 and JP-A-8-44220). What is important in this technique is that a transferring belt must have both the transferring function and the fixing function. That is, because a primary transferring part must have an improved transferring property in a cooled state, and a secondary simultaneous transferring and fixing part must transfer heat at once, a thin layer belt having high heat resistance is used as a material of the belt. Because the transferring efficiency is controlled to an extremely high level, and a large pressure cannot be applied on fixing, a toner is demanded to cope with a low fixing pressure. It is also important that the contamination with the toner on fixing and flaws due to an external additive are minimized as possible on the surface of the belt since the belt also has a transferring function.
Method have been proposed in that high image quality is realized, in particular, a half tone, a solid image and letters are faithfully reproduced, by controlling the volume resistivity of the carrier (JP-A-56-125751, JP-A-62-267766 and JP-A-7-120086). In these methods, the resistivity is controlled by the species of the carrier coating layer and the coating amount, and the objective volume resistivity can be obtained in the initial stage to provide high image quality. However, peeling of the carrier-coating layer occurs due to the stress in the developing device, and thus the volume resistivity is greatly changed. Therefore, the high image quality cannot be manifested in a long period of time.
Furthermore, a method has been proposed in that the volume resistivity is controlled by adding carbon black to the carrier-coating layer (JP-A-4-40471). The method can suppress the change of the volume resistivity due to peeling of the coating layer. However, an external additive added to the toner or the constitutional component of the toner is adhered on the carrier to change the volume resistivity of the carrier, and therefore it is difficult to manifest high image quality in a long period of time as similar to the carrier a described in the foregoing.
The invention has been made to solve the problems associated with the conventional techniques to provide a toner for developing an electrostatic latent image, a process for producing the same, and a developer for developing an electrostatic latent image using the same, which have the following features, i.e., the toner flowability, the charging property, the developing property, the transferring property and the fixing property are simultaneously satisfied in a long period of time; a blade cleaning step accelerating the wear of a latent image holding member is not employed; and the residual transferred toner is recovered simultaneously with the development, or the residual toner remaining on the latent image holding member is recovered by an electrostatic brush. The invention also provides a process for forming an image, in which development, transferring and fixing that cope with the demand of high image quality can be conducted.
As a result of earnest investigation made by the inventors, the problems described in the foregoing can be solved by using a specific monodisperse inorganic oxide as an external additive of a toner, so as to complete the invention.
The invention relates to, as a first aspect, a toner for developing an electrostatic latent image comprising a colored particles containing a binder resin, a coloring agent and a releasing agent, and an external additive, the external additive containing a monodisperse spherical inorganic oxide having a true specific gravity of from 1.3 to 1.9 and a volume average particle diameter of from 80 to 300 nm.
In the toner for developing an electrostatic latent image of the first aspect, it is preferred that the inorganic oxide is silica.
In the toner for developing an electrostatic latent image of the first aspect, it is preferred that the colored particles have a shape coefficient represented by the following equation of 125 or less:
Shape coefficient of colored particles=(R2/S)xc2x7(xcfx80/4)xc2x7100
wherein R represents a maximum length of a diameter of the colored particles, and S represents a projected area of the colored particles.
In the toner for developing an electrostatic latent image of the first aspect, it is preferred that the external additive further contains a reaction product of metatitanic acid and a coupling agent, which has an electric resistance of 1010 xcexa9.cm or more.
In the toner for developing an electrostatic latent image of the first aspect, it is preferred that the monodisperse spherical inorganic oxide is added in an amount of from 0.5 to 5 parts by weight per 100 parts by weight of the colored particles.
The invention also relates to, as a second aspect, a developer for developing an electrostatic latent image containing a toner for developing an electrostatic latent image of the first aspect of the invention and a carrier.
In the developer for developing an electrostatic latent image of the second aspect, it is preferred that the carrier contains a core material covered with a resin coating layer.
In the developer for developing an electrostatic latent image of the second aspect, it is preferred that the carrier contains a core material covered with a resin coating layer containing a matrix resin having a conductive material dispersed therein.
In the developer for developing an electrostatic latent image of the second aspect, it is preferred that the carrier has a shape coefficient represented by the following equation of 120 or less, a true specific gravity of from 3 to 4, and a saturation magnetization at 5 kOe of 60 emu/g or more:
Shape coefficient of carrier=(Rxe2x80x22/Sxe2x80x2)xc2x7(xcfx80/4)xc2x7100
wherein Rxe2x80x2 represents a maximum length of a diameter of the carrier, and S represents a projected area of the carrier.
In the developer for developing an electrostatic latent image of the second aspect, it is preferred that the carrier has a volume resistivity of from 106 to 1014 xcexa9.cm on application of an electric field of 1,000 V/cm.
In the developer for developing an electrostatic latent image of the second aspect, it is preferred that the core material of the carrier is a magnetic powder dispersion type spherical core produced by a polymerization method.
In the developer for developing an electrostatic latent image of the second aspect, it is preferred that the carrier contains magnetic powder in the form of fine particles in an amount of 80% by weight based on the total weight of the carrier.
The invention also relates to, as a third aspect, a process for forming an image containing a step of developing an electrostatic latent image formed on a latent image holding member with a toner to form a toner image, and a step of transferring the toner image to a transferring material to form a transferred image, the toner being a toner for developing an electrostatic latent image of the first aspect of the invention.
In the process for forming an image of the third aspect, it is preferred that the toner image contains color toner images of respective colors, the transferring step contains a step of transferring the color toner images of respective colors to a transferring belt or a transferring drum, and then a step of transferring the color toner images of respective colors to a transferring material at a time.
In the process for forming an image of the third aspect, it is preferred that upon transferring the color toner images of respective colors to the transferring material at a time, fixing is conducted simultaneously with transferring.
In the process for forming an image of the third aspect, it is preferred that a residual toner remaining on the latent image holding member is recovered with an electrostatic brush.
In the process for forming an image of the third aspect, it is preferred that a residual toner remaining on the latent image holding member is recovered into a developing device.
While the development and transferring are influenced by the general transporting property of the developer and the electric current on transferring, they are a process of pulling toner particles away from the binding power of a carrier carrying the toner particles, and adhering the same on the object (a latent image holding member or a transferring material), and therefore they are influenced by balance between the electrostatic attracting force and the adhesion force between the toner particles and a charge controlling member or between the toner particles and the latent image holding member. While the balance is difficult to be controlled, the process directly influences the image quality, and when the efficiency thereof is improved, improvement in reliability and power saving by employing no cleaning step are expected. Thus, higher development and transferring properties are demanded in the process. The development and transferring occur when the electrostatic attracting force is larger than the adhesion force. Therefore, in order to improve the efficiency of the development and transferring, the electrostatic attracting force is increased (i.e., the development and transferring power is increased), or the adhesion force is decreased. In the case of increasing the development and transferring force, when the transferring electric field is increased, for example, a secondary fault, such as formation of an inversely polarized toner, is liable to occur. Therefore, it is more effective to decrease the adhesion force.
The adhesion force includes a Van der Waals force (non-electrostatic force) and an image force by an electric charge of the colored particles. There is a difference of substantially one order between the forces, and the adhesion force can be discussed only by a Van dar Waals force. The Van dar Waals force F between spherical particles can be expressed by the following equation:
F=Hxc2x7r1xc2x7r2/6(r1+r2)xc2x7a2
wherein H is a constant, r1 and r2 are radii of the particles in contact with each other, and a is a distance between the particles. In order to reduce the adhesion force, it is effective to increase the distance a and to decrease the contact area (number of contact points) by intervening fine particles having a radius extremely smaller than the colored particles between the colored particles and the surface of the latent image holding member or the surface of the charge controlling member. The effect can be stably maintained by using the monodisperse spherical inorganic oxide defined in the invention.